In all aspects of entertainment, hype has been increasing inexorably over the course of the last 30 years or so on which I can legitimately comment. I think he's an actor well capable of delivering and who has delivered top class performances...but he's not that much better than far less celebrated actors...any more than Hugh Grant, for example, is that much worse than far more celebrated actors. There's a tendency from those selling their opinions to simplify and compartmentalise and paint things too much in black and white...because it's easier and an easier sell...and this filters through into 'received wisdom'.
It's a general point...I'm not picking on De Niro. It's also only fair to point out that this is one of the main causes of backlashes and negative revisionism which the over-rated may suffer from.
Also I think he's diluted his 'legacy' a bit in the last 10 or 15 years...but I have a hunch that may be a deliberate move a) to cash in and b) because he doesn't feel comfortable being over-revered.